1. Field of the Invention
The present invention relates to a wire bonder used for wiring in integrated circuit assembling process, and more specifically to an apparatus for forming a bonding ball on a tip end of a bonding wire used in such a wire bonder.
2. Description of Related Art
In integrated circuit assembly processes, after an IC chip has been die-bonded on a package substrate, fine aluminum or gold wires are used to make electrical connection between metallized pads on the IC chip and external terminal leads formed on the package substrate. These wires are bonded one at a time, each wire requiring two bonds to be made. Such wire-bonding techniques include thermocompression bonding (pressure and heat), ultrasonic bonding (friction of wire results in melting and alloying of wire), and thermosonic bonding (a combination of the two processes).
Among these wire bonding methods, the ultrasonic bonding of aluminum (Al) wires is advantageous in that (1) since the Al wires can be bonded at an ambient temperature a thermal influence to the bonding portion is very small; and (2) since Al-Al connection can be realized without insulative intermetallic compound, the bonding portion is reliable. However, the ultrasonic bonding has such disadvantages that since the bonding has directionality, not only the wire bonder has to have a complicated structure, but also the bonding speed is relatively low.
On the other hand, the thermocompression bonding is ordinarily performed by melting a tip end of bonding wire so as to form a metal ball and then depressing the metal ball to a portion to be bonded on an IC chip. Therefore, this method is particularly called "ball bonding" or "nailhead" method. This ball bonding method is advantageous over the ultrasonic bonding method in that the bonding has no directionality, and therefore, workability of bonding is very excellent.
Conventionally, a gold wire is used for the ball bonding. However, the gold needs a high material cost, and since the ball bonding of gold wire has to be performed while maintaining the IC clip at a temperature considerably higher than an ambient temperature, an intermetallic compound is formed in the bonding portion.
In order to eliminate the above problems, there has been proposed to perform the ball bonding by using an aluminum (Al) wire instead of the gold wire. The Al has a much lower and more stable material cost. Furthermore, the Al wire does not form an insulative intermetallic compound between the bonding wire and aluminum external leads which are at present widely used in IC and LSI packages, and also, the Al wire can be bonded at a chip temperature considerably lower than that required for gold ball bonding, namely at a room temperature.
However, the ball bonding of Al wire has one significant problem. Namely, when the Al wire is molten so as to form an Al ball on a tip end of the Al wire, the Al takes part in a chemical reaction with oxygen in atmospheric air, so that the resulting Al ball has not only a rough surface but also a hard coating of oxide formed on the surface. As a result, when the bonding is carried out, electrical connection often becomes defective, or the contact surface of the IC chip is damaged.
In view of these defects, melting of an Al wire tip end in an inert gas atmosphere has been proposed in U.S. Pat. No. 4,387,283, issued to Peterson et al on June 7, 1983. Referring to FIGS. 4 to 6 of this U.S. Patent, the mechanism for forming an Al ball on a tip end of an Al bonding wire includes a capillary tube 51 having a very thin through-hole for supporting and guiding an aluminum wire 53. An electrode 59 is positioned beneath the tip 55 of the capillary tube 51, and a nozzle 49 is located in an inclined condition to inject an inert gas such as argon toward a gap 67 formed between the electrode 59 and the tip end 57 of the Al wire 53.
With this arrangement, an inert gas is injected from the nozzle 49 to form and maintain an inert gas atmosphere around the tip end 57 of the Al wire 53. In this condition, an electric voltage is applied between the capillary tube 51 and the electrode 59, so that an arc discharge takes place between the tip end 57 of the Al wire and the electrode 59. As a result, the heat generated by the arc discharge melts the Al wire to form an Al ball 94 on the Al wire tip end. At this time, the Al wire and the Al ball is prevented from oxidation by the inert gas atmosphere.
In order to perfectly prevent oxidation of the Al bonding bail, it is necessary to perfectly surround the tip end of the Al wire with the inert gas. However, since the periphery of the tip end is an opened space, the inert gas injected towards the tip end cannot be held around the tip end. Therefore, it is necessary to ceaselessly inject a large amount of inert gas to the tip end of the Al wire, so as to maintain a shield of pure inert gas around the tip end. Because of this, an inert gas was injected at the rate of 2000 cm.sup.3 /minute in a typical example. However, if a large amount of inert gas is injected from the nozzle towards the tip end of the Al wire, the formed Al ball 76 is deviated from the center axis 74 of the Al wire as shown in FIG. 2 of U.S. Pat. No. 4,387,283. This is because the injected inert gas flow must be brought into a high pressure for increase of flow amount. If the bonding ball is deviated from the center axis line of the Al wire, the bonding ball cannot often be contacted to a desired point of a contact pad. This is a cause for defective connection. On the other hand, if the injected gas amount is decreased so as to prevent the formed Al ball from being deviated from the Al wire center axis, the atmospheric air will mix with the injected inert gas around the Al wire tip end, so that the Al ball cannot be perfectly protected from oxidation.